The present invention relates to radio communications, and more particularly, to apparatus, methods and computer program products for processing spread spectrum communications signals.
Spread spectrum signal transmission techniques are widely used in communications systems, such as code division multiple access (CDMA) cellular telephone networks. Referring to FIG. 1, an information symbol is typically modulated by a spreading sequence before transmission from a transmitting station 110 such that the symbol is represented by a number of chips in the transmitted signal. At the receiver 120, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code. The receiver 120 including a radio processor 122 that performs downconversion, filtering and/or other operations to produce a baseband signal that is provided to a baseband processor 124. The baseband processor 124 despreads the baseband signal to produce symbol estimates that are provided to an additional processor 126, which may perform additional signal processing operations, such as error correction decoding.
In coherent direct-sequence CDMA (DS-CDMA) systems, coherent RAKE reception is commonly used. This type of receiver despreads the received signal by correlating to the chip sequence to produce despread values that are weightedly combined according to estimated channel coefficients. The weighting can remove the phase rotation of the channel and scale the despread values to provide “soft” values that are indicative of the transmitted symbols.
Multipath propagation of the transmitted signal can lead to time dispersion, which causes multiple resolvable echoes of the transmitted signal to arrive at the receiver. In a conventional RAKE receiver, correlators are typically aligned with selected echoes of the desired signal. Each correlator produces despread values that are weightedly combined as described above. Although a RAKE receiver can be effective in certain circumstances, self and multi-user interference can degrade performance by causing loss of orthogonality between spreading-sequence defined channels.
A “generalized” RAKE (G-RAKE) receiver has been proposed to provide improved performance in such interference environments. A conventional G-RAKE receiver typically uses combining weights that are a function of channel coefficients and a noise covariance that includes information relating to the interfering signals. These weights w may be expressed as:w=R−1c,  (1)where R is a noise covariance matrix and c is a channel coefficient vector.
A typical baseband processor for a G-RAKE receiver is illustrated in FIG. 2. Chip samples are provided to a finger placement unit 230, which determines where to place fingers (selecting delays for one or more antennas) by a correlation unit 210. The correlation unit 210 despreads one or more traffic channels and produces traffic despread values. The selected paths are also provided to a weight computer 240, which computes combining weights that are used to combine the despread values in a combiner 220 to produce soft values.
Similar functionality may be provided using a chip equalizer structure, as shown in FIG. 3. In such a structure, chip samples are provided to a tap placement unit 330, which determines where to place filter taps (i.e., which delays for one or more antennas) for a finite impulse response (FIR) filter 310. The selected tap locations are also provided to a weight calculator 340 that computes filter coefficients (or weights) for the filter 310. The filter 310 filters the chip samples to produce a signal that is despread by a correlator 320 to produce symbol estimates.
A conventional weight computer for a G-RAKE receiver is illustrated in FIG. 4. Signal samples are provided to a correlation unit 410 that despreads symbols from a pilot or traffic channel to produce initial despread values. Symbol modulation is removed from these values by a modulation remover 420, and the resulting values are provided to a channel tracker 430 that generates channel estimates. The despread values and channel estimates are provided to a noise covariance estimator 450, which produces an estimate of the noise covariance of the set of delays in use. The channel estimates and noise covariance estimate are provided to a weight calculator 440, which computes combining weights (filter coefficients) therefrom.
A G-RAKE receiver differs from a traditional RAKE receiver in that it considers delays in addition to those corresponding to echoes of the desired signal. These other delays are typically chosen to provide information about interference so that the receiver may suppress the interference.
In a practical RAKE receiver (traditional RAKE or G-RAKE), hardware and/or software constraints typically limit the number of “fingers” that can be used at any given time. In a traditional RAKE receiver, these fingers are typically chosen such that a maximum amount of the desired signal's energy is collected. In a G-RAKE receiver, however, the finger selection criteria also may collect interference signal information such that a desired amount of interference suppression can be achieved.
A variety of strategies for selecting fingers for RAKE receivers have been proposed. U.S. Pat. No. 5,572,552 to Dent et al. describes a process whereby fingers are placed according to a signal to noise ratio (SNR) metric that is computed as a function of channel coefficients, power levels and, optionally, the spreading code. U.S. Pat. No. 6,363,104 to Bottomley describes estimating SNR for different combinations of finger positions as a function of channel estimates and impairment correlation matrix estimates for each candidate combination, and selecting a finger combination that maximizes SNR. U.S. Pat. No. 6,683,924 to Ottosson et al. describes a finger selection process based on time differentials and relative signals strengths of signal paths. “Low complexity implementation of a downlink CDMA generalized RAKE receiver,” and “On the performance of a practical downlink CDMA generalized RAKE receiver,” by Kutz et al., Proc. IEEE Veh. Technol. Conf., Vancouver, Canada (Sep. 24-28, 2002), describe other selection techniques.